CA1150076A - Diaphragm for pressure sensors - Google Patents

Abstract

DIAPHRAGM FOR PRESSURE SENSORS

ABSTRACT

A diaphragm consisting of multiple arches con-figured such that radial compliance is increased and pres-sure responsive radial compression loading will approximate radial tension loading resulting from deflection of an associated pressure sensor mechanism. On the pressure receiving side of the diaphragm, a broad shallow convex section extends between two concave sections of tighter curvature. The portions of the concave sections remote from the convex section each extend to a substantially cylindrical configuration. A sharp convex bend extending to a flat radial flange provides an attachment edge with minimum attachment stress. A method of forming such a diaphragm to insure uniform wall thickness by allowing maximum lateral migration of the diaphragm sheet is also disclosed.

T-2643-239-1 :

Description

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BACKGROUND OF THE INVENTION
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The presen~ invention relates to diaphragms for pressure sensors and the method OI forming same.
~his application is a diYisional of Canadian Patent Application SoN~ 333~006 filed August 1, 19790 Various configurations of diaphragms have been used in a wide variety of pressure sensor devices with th~ primary ob~ect of these diaphragms being to seal the inside of the devlce from the surrounding environment while all4win~ the force collector $o move in response to the measurecl pressure. To thi~ end, metal sheet-like elements generally of circular configuration have been used.
The contour of such diaphragms is also generally symmetri-cal about a central axis with concentric, convex and con-cave deformations in the sheet. The deformations act to reduce the force re9uired to bend the diaphragm in response to movement of the central force collecting piston of the pressure sensor~ The deformations also are intended to relieve radial stress in the diaphragm as the ~orce collect-ing piston moves axially under pressure load.
The r~sistance which is encountered with such ~iaphragms is generally non-linear. This is in part because the radial strain experienced by such diaphragms is related to ~he axial movement o~ the force collecting piston by the - - approximated relationship that the elongated radial width of the active portion of the diaphragm is equal to the square root of the sum of the s~uares of t~e relaxed radial wiath of the active portion of the diaphgram and the axial dis-placement of the force collecting piston~ Such a non-linear relationship affecting the performance o~ the diaphragm ~ , T~2643-239-1 ~ 1 -~is~C!7~
results in a n~n-linear response of the sensor to pressure.
Calibration of the instrument over its operating range.
Another difficulty encountered by thin metal diaphragms is stress concentration at both the annular mounting rim o~ th~ senso~ and the force collecting piston.
5 Diaphragm failure is frequently experienced at these points rather than at some intermediatP point therebetween. The abrupt change from the flexible unsupported portion of the diaphragm to the rigid components of the sensor is primarily responsible for these problems.
In attempting to overcome the foregoin~ diffi-culties, i.e., a non-linear spring rate of the diaphragm and high stress loadings at the attachment poînts, various configurations have previously been employed. ~owever, efforts at solving these problems have generally resulted 15 in an improvement in diaphragm performance with respect to one consideration at the expense of the other consideration.
Consequently, no real solution to the improvement of over-all diaphragm performance has heretofore been found~

! ' ~0 SUMMARY OF THE I~VENTTON
The present invention is directed to improved diaphragm configurations for pressure sensor~ and the methods of forming same. Through the present invention, the non-linear radial resistance of a diaphragm can be greatly - 25 reduced as well as the high stress concentrations at the attachment points7 The special configuration of the present invention can be advantageously formed through a two-step process without undue stretching of the diaphragm material.

~l~5~3~j To accompli~h the substantial reduction in non-linear radial resistance of the diaphragm, a specific radial profile has been developed which includes, as viewed from the pressure receiving side of the diaphragm, a broad~ convex section associatea at ~he inner and outer edges thereof with concave sections having smaller radii of curvature than the broad, convex,portion. The concave sections extend away from the convex section to portions ~hich approach a perpendicular orientation relative to the plane of the diaphragm. This 1~ orientation ma~ be characterized as creating substantially cylindrical portion~.
From the foregoing configuration, two effects are obtained. E`irst, the broad, convex portion will tend to flatten out with increasing pressure. This results in com-pression loading on the concave sections. As pressure in-creases and as the center convex sec~ion tends to flatten out, the force collecting piston of the transducer moves axiallyO
This axial movement results in a radial tension loading of the ~iaphrasm. Through empirical analysis, diaphragms can be developed using the present configuration which tend to offset the compression loading of the convex portion with the tension loading caused by the axial movement of the force collecting piston. The concave sections add significantly to the reduction in flexure rigidity of the diaphragm and provide little resistance ~o the flattening of the center convex section. Also, the broad, convex section provides increased raaial compliance irrespectiYe of its pressure responsive characteristic as compared to the sinesoidal convolutions o~
conventional diaphragms. Thus, the non-linear resistance to the required radial extension of the diaphragm can be sub-stantially overcome.

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Second,stresses at the attachment points of the diaphra9m are also reduced by the configuration of the present inven~ion- As stated above, the concave sectionS of the diaphragm extend upwardly to substantially 5 cylindrical portions. Once having approached such a cylindrical configuration, a bend may be formed in the diaphragm which is preferably about a~minimum radius of curvature for the material of the diaphragm. From this bend on each edge of the diaphragm, a radially extending 10 flange is provided. The flange provides an area for attachment of the diaphragm to the pressure sensor.
Finally, the diaphragm is welded or otherwise fixed to the pressure sensor at the flanges of the diaphragm as closely as possible to the sharp bends of the diaphragm.
15 Alternately, the diaphragm may be directly welded to the pressure sensor where the concave sections approach a substantially cylindrical configuration The substan-tially cylindrical configuration provides great rigidity ; to the diaphragm at that location. The rigidity is such 20 that it overcomes any stress concentration which might otherwise be transmitted to the attachment area. Instead, the other portions of the diaphragm between the generally cylindrical portions will take the strain. Naturally, the sharp bend also provides added rigidity to the flange and 25 attachment points.
In order to form the sharp bend and maintain a relatively uniform wall thickness, across the width of the diaphragm a method has been developèd by the present in-vention which avoids formation of the sharp be~d during 30 formation of the convex and concave annular sections.

During the formation of these sections, the annular sheet of metal being formed into the diaphragm tends to migrate radially inwardly from the oute~ edge and radially out-wardly from the inner edge in defining the annular rings.
If the sharp bend is to be forTned in the same process, 5 the dies tend to hold onto both the inner and outer edges of the annular sheet. Under such circumstances, the center portions of the sh~et may tend o stretch exces-sively, leading to a non-unifor~ wall thickness of the resulting diaphragm. By the present invention, the sharp 10 bends formed at the inner and outer edges of the diaphragm are completed by a second forming step.
Accordingly, it is an object of the present in-vention to provide an improved diaphragm for pressure sensing devices.
It is another object of the present invention to prov~de a diaphragm for pressure sensors having mini-mum resistance to the operation of the sensor.
It is yet another object of the present inven-tion to provide a diaphragm for pressure sensors having 20 low stress concentrations at the attachment points of the diaphragm.
It is yet another object of the present inven-tion to provide an improved method for forming diaphragms of the present invention.
Other and further objects and advantages will appear ~ereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a cross-sectional elevation of a 30 diaphragm of the present invention shown in place in a ~ SC~076.
pressure sensor.
Figure 2 is a cross-sectional elevation of a detail of the pressure sensor of Figure 1.
Figure 3 is a cross-sectional elevation of a sezond embodiment of the diaphragm showing attachment to the pressure sensor~ - -Figure 4 is a graph showing the percent devia-tion in resistance force of the diaphragm with increasing pressure.
10Figure 5 is another embodiment of a diaphragm of the present invention shown in cross-sectional eleva-tion.
Figure 6 is a second application shown in cross-sectional elevation of the diaphragm of Figure 1.
1~Figure 7 is a third application shown in cross-sectional elevation of the diaphragm of Figure 1.
Figure 8 is a cross-sectional elevation of a die used in the first step of forming a diaphragm of the pres-ent invention.
20Figure 9 is a die used in the second step of forminy a diaphragm of the present invention.

DETAILED_DES~RIPTION OF THE PREFERRED EMBODIMENT
Turning in detail to the drawings and in parti-cular the embodiment illustrated in Figures 1 and 2, a 25 portion of a pressure sensor is disclosed. The term "pressure sensor" as used herein is any device which is responsive to pressure by having the sensed pressure operate to deflect a piston resistive to such deflection from its rest position. As illustrated, the pressure sen-30 sor may have an annular rim 10 ~or attachment to the outer edge of a diaphragm. The rim may or may not have a step in ~L5~0~
the upper surface thereof to insure proper pl~cement of the diaphra9m on the rim- A force collecting piston 12 is shown to be concentrically mounted relative to the annular rim lO at approximately the same level. A shaft 5 14 extends to the recording portion of the sensor.
A diaphragm, generally designated 16, is shown here to be annular in overall plan. As may be preferred in some applications, the diaphragm may not have a central hole therethrough. The presence or absence of a central 10 hole in the diaphragm is of little consequence to the overall operation of the annular portion or element which constitutes the active part of the diaphragm.
The operation of the pressure sensor re~uires that the piston 12 move downwaldly in response to in-15 creased pressure on the outside of the sensor. Resis-tance to movemen~ of the piston 12 is usually provided by a spring or other mechanism which exhibits a substantially constant ratio between displacement and resisting force.
In other words, the resistance provided by the sensor it-20 self generally is designed to exhi~it a fixed spring con-stant. At the same time, the diaphragm is designed to provide as little resist~nce to movement of the piston 12 as possible. Diaphragms are also generally designed to ex-perience a minimum amount of sag between their inner and 25 outer support. For convenience the convexity and con-_ cavity of the diaphragm`will be defined here in terms of the surface which is on the pressure side of the diaphragm.
Naturally, as the diaphragm is of generally uniform thick-ness, these features will be reversed on the sensor side 30 of the diaphragm.

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The diaphragm of the preferred emb~diment as seen in Figure 1 incorporating the present invention em-ploys a hroad, relatively shall~w, convex span 18 forming a central annular section. Radially in either direction 5 from convex span 18 are shorter, deeper, con~ave spans 20 and 22. The annular concave spans ~0 and 22 continue to substantially cylindrical portions 24 and 26~ In the pre-ferred embodiment of Figure 1, a sharp, convex bend ~8 and 30 extends from each substantially cylindrical portion 24 10 and 26 respectively. The sharp, convex bends 28 and 30 are preferably about a minimum radius for the material empl~yed as the diaphragm. Lastly, attachment flan~es 32 and 34 extend from the sharp, convex bends 2~ and 30 in radial directions for association with the piston 12 and the 15 annular rim 10 of the pressure sensor. In the alternate embodiment af Figure 3, attachment is made to the inner surface of the annular rim 10; and therefore, the sharp, convex bends 28 and 30 and the att:achment flanges 32 and 34 become unnecessary.
,. ~0TQ achie~e a minimum effect of the diaphragm on the overall resistance to movement of the force collect-ing piston 12, counter-balancing mechanisms have been de-signed into the diaphragm of the present invention. To this end, an effort has been made to minimize the resist-25 ance to radial extension of the diaphragm required as the - force collecting piston 12 moves axially and hence away from the annular rim 10. Secondly, the diaphragm of the present invention has been designed to itself respond to the accumulated pressure on the press~re side ~f the sen-30 sor. This pressure response of the diaphragm causes radial expansion of the diaphragm to keep up with t~e ~5~7~6 reguired elongation as the central force collecting piston 12 moveS under the accumulated pressure. The concave sec-tions 20 and 22 minimize bending stresses by providing an extended length over which bendiny stra~s may occur; and the convex section exhibits substantial radial compliance.
Thus, these sections provide maximum flexibility of the diaphragm in the required directions. The central, convex span 18 is relatively broad and shallow in order that it will respond to pressure on the pressure side of the sen-sor. As pressure is increased on the diaphragm, theconvex span lB will tend to flatten. This flattening of the span 18 will result in radial compression loading or outward movement of the concave spans 20 and 22. At the same time, as pressure increases on the sensor, the piston 12 will tend to move axially. This movement will require a radial elongation of the diaphragm 16. Thus, convex span 18 will act to meet that requirement by itself being flat-tened by the pressure. The concave sections minimize the bending required in accommodating bo$h the extension of the

2~ diaphragm and the flattening of the convex portion thereof.
~ either the flattening of the convex portion of the diaphragm nor the overall radial extension of the diaphragm are linear functions. Consequently, the con-siderations necessary in calculating the appropriate dimensions for such a diaphragm become prohibitively com-plicated. Through empirical testing, appropriate relation-ships can be found for any given diaphragm size normally associated with such pressure sensors. Tests have demon-strated that certain approxi~ate relationships can be employed as a basis from which to start empirically fine 7~

tuning the design of a diaphra~m to fully realize the advantages of the present invention. Tne diaphra~ms which have been tested are of the type suitable for use in sensorS where eighty to ninety percent of the systems 5 stiffness is in a force gaging device such as a strain gage equiped cantilever beam~ Only ten to twenty per-cent of the stif~ness is in the diaphragm.
In ~escribing SUCh relationships, certain de~initions are necessary~ The active span of the dia-10 phragm oi the preferred embodiment is the distance betweenthe two cylindrical portions 24 and 26. It is within this area that s~bstantially all of the diaphragm flexure occurs.
l~hrough empirical methods, a first a~proximate radius of curvature ~or the two concave sections 20 ana 22 has been 15 found to be around one eighth of the active span length. A
~irst approximate radius of curvature of the convex span 18 nas been found to be acceptable when approximately equal to the active span length. When properly matched and empiri-cally tuned, a response curve of the overall sensor may be 20 arrived at such as shown in Figure 4. In Figure 4, the deviation of the spring constant from its constant value is shown across the full range of pressure for the pressure sensor. Naturally, hysteresis inevitably affects the per-formance of such a device but the device may be adjusted to 25 give zero deviation at zero pressure and maximum pressure as shown.
To eliminate concentration ~f bending stresses at the attachment points of the diaphragm, the concave portions 20 and 22 extend up to substantially cylindrical portions 24 30 and 26. Ihese portions 24 and 26 act to resist bending and radial stresses and thus do not transmit such stresses :

~5~7~6 through to the attachment points of the diaphragm. In the case of the embo~iment as shown in Figure 3/ this effect oE
the cylindrical portion is used exclusively to reduce bend-ing load on both the diaphragm adjacent the attachment point 5 and the bond itself. In Figure 3, the diaphragm is shown to be welded to the inside of the annular rim 10 at 36.
In Figure 2, the cylindrical portions 24 and 26 are employed with the sharp, convex bends 28.and 30 and the radial attachment flanges 32 and.34 to provide riqid resist-ance to transmission of bendin~ stresses directly to theattachment locationO It is also advantageous that the attachment.point shown as weld 38 is as close as pract-ical to the inner edge of the rim 10. This prevents any cantilevering effect of the diaphragm at the inner corner of the rim 10. Resistance to bending is maximized in the sharp, convex bend 28 and 30 of the diaphragm by having the radius of curvature approach the minimum possible for the diaphragm material and thickness used. As the thickness of such diaphragms generally ranges from .0015 to .0110 inches, the radius of this sharp bend may preferably range from .003 inches to .010 inches. Through the use of the concave sections 20 and 22 with the cylindrical portions 24 and 26 and the sharp bends 28 and 30/ maximum compliance is obtained with minimum stress concentration. As a result, the pressure responsive characteristic of the convex span 18 may be allowed to operate as freely as possible to accommo-date the necessary radial extension of the diaphragm under loading.
As a variation on the present embodiment, a plur-ality of diaphragm elements are disclosed in Figure 5 It can be seen from Figure 5 that a plurality of complete elementS having the concave sections 20a and 22a and the convex sectio~ 18a may be used to arrive at the same result as the single element shown in Fi~ure 1. The diaphragm 16a of Figure 5 iS shown to be positioned between annular rim 5 lOa and force collecting piston 12a.
Two uses for the diaphragm as illustrated in Figure 1 are shown in Figures 6 and 7. In each of these - Figures, diaphragms are placed in series to accomplish greater axial flexibility of the instrument. As both the 10 outer rims 40 and 42 and the inner elements 44 through 52 do not expand or contract radially, the same compensa~ing effect of the flattening of the central convex span 18 is required. It should be noted tha~ not all of the dia-phragms in Figure 7 are oriented to have the convex sec-1~ tion face the pressure input. Instead, the system isbalanced to provlde two in eacll direction, making the device reversible with maximum compliance provided by the present diaphragm co~figuration.
Figures 8 and 9 show th~e manufacture of a dia-20 phragm of the present inven~ion. In Figure 8, a first dieis employed to define the complete proile of the diaphragm inside of the two sharp, convex bends which exist at the inner and outer edges of the diaphragm when completed. A
more moderate bend is made. Shown in phantom in Figure 8 25 is the original sheet prior to forming. As can be seen, substantial migration of the metal occurs in both radial directions with the formation of the central portion of the diaphragm.
In Figure 9, a second die 56 is shown which is used 30 to simply form the outer, sharp convex bends 28 and 30 once 7~
mi9ration of the sheet material into`the central portion of the die is no longer a consideration, This two step pro-cedure has been found to make more uniform the overall thickneSS and sonstruction of the diaphragm when complete.
Thus, a diaphragm and method of making same are 5 disclosed herein which haYe the ability to provide a minimum e~fect on the spring constant of the pressure sensor, re-duce bending stresses at the attachment points of the dia-phragm and make more uniform the overall diaphragm construc-struction. While embodiments and applications of this in-10 vention have been shown and described, it would be apparent to those skilled in the art that many more modifications are possible without departing from the inventive concepts herein described. The invention, therefore, is not to be restricted except by the spirit of the appended claims.

Claims (2)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY
OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An annular diaphragm assembly of a pressure sensor, comprising, a circular support rim, a concentric disc and a first sheet like diaphragm member having a pressure receiving side, said pressure receiving side having a first annular concave section extending inwardly to form a first substantially cyl-indrical portion, a second annular concave section radially outwardly and spaced from said first annular concave section extending outwardly to form a second substantially cylindrical portion, an intermediate section extending between said first and said second annular concave sections, a first annular, sharply convex section radially inwardly of said first annular concave section at said first substantially cylindrical portion thereof, a second annular, sharply convex section radially outwardly of said second annular concave section at said second substantially cylindrical portion thereof, a first flat sensor attachment flange extending inwardly from said first annular, sharply convex section, said first flat sensor attachment flange being attached to said disc immediately adjacent the peripheral edge thereof, a second flat sensor attachment flange extending outwardly from said second annular, sharply convex section, said second flat sensor attachment flange being attached to said rim immediately adjacent the inner edge thereof.

2. A method of forming an annular diaphragm having a substantially right angle bend about a minimum radius to a flat radially extending flange at each of the inner and outer edges of said diaphragm, comprising the steps in sequence of forming in a die an annular seat to the final configuration of the diaphragm between the right angle bends at the inner and outer edge of the diaphragm while allowing the inner and outer edges of the sheet to migrate laterally within the die; and forming in a die a substantially right angle bend about a minimum radius and a flat radially extending flange at each of the inner and outer edges of the diaphragm.